GK Chesterton put it like this:

And now, after a due pause, someone will ask and we must answer a popular question which, like many popular questions, is really a somewhat deep and subtle one. To put the matter quite simply, as the popular instinct would put it, ‘Was William Blake mad?’

Alexander Gilchrist, Blake’s first biographer, dedicated an entire chapter of his book Life of William Blake to the subject, a chapter entitled ‘Mad or Not Mad’. On reading Gilchrist, it seems that the principle evidence for Blake’s madness was his habit of speaking matter-of-factly about spiritual visitors. He would casually drop into conversation that he had been speaking to Milton, or any other dead poets or kings that might occur to him, and elaborate on precisely what they had discussed. He also proclaimed he saw romantic visions, such as sculptures of sheep in the middle of a field where there were none. WC Dandy in his book The Philosophy of Mystery (1841) summed up the 19th century view of this behaviour thus:

The difference between Shakespeare and Blake is antipodean. Blake was a visionary and thought his fancies real – he was mad. Shakespeare was a philosopher, and knew all his fancy was but imagination, however real might be the facts he wrought from them.

But Gilchrist describes these visions of Blake’s as the peculiar power of a creative artist of genius – a power that was fully under his own control:

According to his own explanation, Blake saw spiritual appearances by the exercise of a special faculty – that of imagination – using the word in the then unusual, but true sense, of a faculty which busies itself with the subtler realities, not with fictions. He, on this ground, objected even to Shakespeare’s expression:

And gives to airy nothing
A local habitation and a name

He said the things imagination saw were as much realities as were gross and tangible facts. He would tell his artist friends: ‘You have the same faculty as I (the visionary), only you do not trust or cultivate it. You can see what I do, if you choose.’ In a similar spirit was his advice to a young painter: ‘You have only to work up imagination to the state of vision, and the thing is done.’ After all, he did but use the word vision in precisely the same sense in which Wordsworth uses it to designate the poet’s special endowment.

Whatever the truth of this, many of Blake’s contemporaries remarked on the appearance of his madness. Robert Southey, in a letter to Caroline Boules (1830) said:

Much as he is to be admired, he was at that time so evidently insane, that the predominant feeling in conversing with him, or even looking at him, could only be sorrow and compassion… You could not have delighted in him – his madness was too evident, too fearful. It gave his eyes an expression such as you would expect to see in one who was possessed.

But other contemporaries held contrary opinions. James Ward, who had often met Blake in society and talked with him, would never hear him called mad. Likewise, his fellow artist Edward Calvert said: ‘I saw nothing but sanity. Saw nothing mad in his conduct, actions or character.’ And another contemporary, one Mr Finch, summed up his recollections thus: ‘He was not mad, but perverse and willful.’

Judging by Seymour Kirkup’s reflections, it was never an easy case to settle:

I was with him from 1810 to 1816, when I came abroad… His high qualities I did not prize at that time; besides I thought him mad. I do not think so now!

GK Chesterton, who denies that the stories of visions were enough to prove him mad, paints a fascinating picture of Blake’s extraordinary mind:

Originally his mind was not only strong, but strongly rational – one might almost say strongly sceptical. His mind was like a ruined Roman arch; it has been broken by barbarians; but what there is of it is Roman. In his art criticism he never said anything that was not strictly consistent with his first principles. In his controversies, in the many matters which he argued angrily or venomously, he never lost the thread of the argument. But something, when all is said and done, had eaten away whole parts of that powerful brain, leaving parts of it standing like great Greek pillars in a desert.

Where he does locate a kind of madness in Blake, is in an ‘actual and abrupt irrelevance’ in his poetry.

He had in his poetry one very peculiar habit, a habit which cannot be considered quite sane. It was the habit of being haunted, one may say hag-ridden by a fixed phrase, which gets itself written in ten separate poems on quite different subjects, when it had no apparent connection with any of them. The harmless Hayley [one of Blake’s patrons]… provoked Blake’s indignation by giving him a commission for miniatures when he wanted to do something else, probably frescoes as big as the house. Blake wrote the epigram –

If Hayley knows the thing you cannot do,
That is the very thing he’ll set you to.

And then, feeling that there was a lack of colour and warmth in the portrait, he lightly added, for no reason in particular, the lines –

And when he could not act upon my wife,
Hired a villain to bereave my life.

There is, apparently, no trace here of any allusion to fact. Hayley never tried to bereave anybody’s life. He lacked even the adequate energy. But now turn to another poem of Blake’s, a merely romantic and narrative poem called Fair Eleanor, which is all about somebody acting on somebody else’s wife. Here we find the same line repeated word for word in quite another connection –

Hired a villain to bereave my life.

It is not a musical line; it does not resemble English grammar to any great extent. Yet Blake is somehow forced to put it into an utterly different poem about a fictitious person. There seems no particular reason for writing it even once; but he has to write it again and again. This is what I do call a mad spot on the mind…

In four or five different poems, without any apparent connection with those poems, occur these two extraordinary lines –

The caterpillar on the leaf
Repeats to thee thy mother’s grief

In the abstract this might perhaps mean something, though it would, I think, take most people some time to see what it could mean. In the abstract it may perhaps involve some allusion to a universal law of sacrifice in nature. In the concrete – that is, in the context – it involves no allusion to anything in heaven or earth. Here is another couplet that constantly recurs –

The red blood ran from the grey monk’s side,
His hands and his feet were wounded wide.

This is worse still; for this cannot be merely abstract. The ordinary rational reader will naturally exclaim at last, with a not unnatural explosion, ‘Who the devil is the grey monk? And why should he be always bleeding in places where he has no business?’ Now to say that this sort of thing is not insanity of some kind is simply to play the fool with the words. A madman who writes this may be higher than ordinary humanity; so may any madman in Hanwell. But he is a madman in every sense that the word has among men.

Whatever our view on Blake’s madness, we must surely concur with Wordsworth:

There was no doubt that this poor man was mad, but there is something in the madness of this man which interests me more than the sanity of Lord Byron and Walter Scott.